The radian is a unit which connects the radius of an arc, the length of the arc and the angle subtended by the arc.

The most common geometry for curved crystal diffraction is based on the Rowland circle principle, which relies on the well-known property of a circle that an arc segment subtends a constant angle for any point on the circle.

Reticles are in the second focal plane, so as power is changed the angle subtended by the space between lines varies.